All depends on what the light was. Was it florescent or incandescent? If incandescent, then the light is almost useless. You need a screwin CF bulb, with a 6500k rating. On a 10G, I have one that has 2 20W bulbs, and another that has 2 25W bulbs, and the one with the 25W bulbs need CO2. The tank with just the 20W bulbs is algae free, and no CO2. So if you have a lamp over the tank with a single bulb, I would recommend getting at least a 25W bulb, as it's the only bulb over the tank. You won't need CO2.

Now if you are wanting a grassy type of plant, you will need higher light than that, as well as adding CO2. Most ground covers require higher lighting.